Method for making syntactic foam



' Nov. 17, 1970 l. REsNlcK l METHOD FOR MAKING SYNTACTIC FOAM OriginalFiled March 2a, 196e 2 sheets-sheet i wm wm s mm. wm um mm n QQN Nxm

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- Nov. 17, 1970 1. REsNlcK METHOD FOR MAKING SYNTACTIC FOAM OriginalFiled March 28, 1966 TEMPERHrZ/ef 60A/mouw l I l l l l I I I I I l l I Il I I l I I l l l l I l l I l l l n l OVE/Vgn United States Patent O3,541,194 METHOD FOR MAKING SYNTACTIC FOAM Israel Resnick, Bellerose,N.Y., assignor to the United States of America as represented by theSecretary of the Navy Original application Mar. 28, 1966, Ser. No.538,920, now Patent No. 3,477,967, dated Nov. 11, 1969. Divided and thisapplication Oct. 15, 1968, Ser. No. 798,823

Int. Cl. B2Sb 1/.08; C08f 47/10; C08g 53/08 U.S. Cl. 264-71 2 ClaimsABSTRACT OF THE DISCLOSURE A method of preparing syntactic foam buoyancymaterial is disclosed including confining and compacting a quantity oflow density iiller, flowing an uncured epoxy resin mix through thecompacted iiller by drawing the said mix upward by means of a vacuumpump attached to the mold confining the iiller to fill the entire volumeof the mold not occupied by the said filler and curing the epoxy resinmix.

This application is a division of U.S. application Ser. No. 538,920,filed Mar. 28, 1966 for Syntactic foam, now Pat. No. 3,477,967.

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

This invention relates to buoyancy materials for general applicationsbut more particularlyfor deep sea applications.

Better buoyancy materials are needed for incorporating in payloadtransporting deep submergence buoyant vehicles and are also needed forsupporting tethered ocean platforms, acoustic arrays, and underwaterequipments above the bottom. For some applications, it would beadvantageous for the buoyancy material to be a sound attenuator. Anideal deep submergence buoyancy material would have substantial netbuoyancy in terms of its weight and volume, minimum cost per pound ofnet buoyancy,

such high compressive strength as to suffer no damage even in thedeepest places in the oceans, zero water absorption, shall suffer nodeterioration in compressive strength nor absorb water whether subjectedto continuous high pressure or cyclic changes in pressure ranging'between atmospheric and that at the ocean door, essentially zerotemperature coeicient of expansion and constraction, adequate impactstrength, and bulk modulus of compressibility equal to or higher thanthat of water. The latter is dened as the ratio of pressure tocompressibility; compressibility is the change in Volume (over thepressure range), divided by the volume.

Attention was focused on buoyancy material for deep submergence vehicleswhen the bathyscaphe Trieste made its now historic dive. The Trieste canbe compared to a blimp in the sense that both have a pod appended to acomparatively large buoyancy tank. The buoyancy tank of the Trieste wasdesigned to contain gasoline for want of a better buoyancy materialdespite serious disadvantages including fire hazard, substantialcompressibility affecting buoyancy and trim stability, significantcontraction With drop in temperature, and the danger accompanying theuse of any tluid for buoyancy, namely, escape of the 3,541,194 PatentedNov. 17, 1970 icc tluid through any fault in the Wall of the buoyancytank. Furthermore, buoyancy tanks suitable for the pressure range arevery expensive.

Metallic lithium with a density of 33 lbs. per cubic foot is one of theleast dense solids available. It has been considered for use as a deepsea submergence buoyancy material. Because lithium and water reactvigorously when brought together, liberating hydrogen gas, this materialhas been deemed too hazardous for personnel carrying vehicles. Suitablecorrosion resistant metallic containers for lithium significantly reducethe net buoyancy of the combination. Also, the cost of lithium plus thesuitable container is too high.

Other materials which have been considered include wood, organicpolymers including polyethylene and polypropylene, foamed plastics, andmetal or ceramic foams. Wood as a buoyancy material is limited tosurface or near surface applications because of low strength and highwater absorption. Foams, both plastic and inorganic, have excessivepermeability and inadequate strength for deep submergenc. Low densityplastics have limited buoyancy as a class. At moderate depth, plasticsin the form of thickwalled hollow spheres are useful but for deepsubmergence applications, they are unsuitable.

An object of this invention is to provide a buoyancy material as nearideal as possible for deep submergence applications.

A further object is to provide a buoyancy material suitable for longterm use under hydrostatic pressure greater than 10,000 pounds per square inch.

A further object is to provide a reliable buoyancy material for longterm use in a deep submergence vehicle that may range between thesurface and thousands of feet in depth and that can also providestructural support to the hull of the vehicle.

A further object is to provide superior deep-depth buoys for reliablelong-term use as cable, platform, acousticarray or equipment supports.

A further object is to provide a superior buoyant material for long termuse under water at great depth and which has sound attenuationcharacteristics as well as superior buoyancy characteristics.

A further object is to provide a buoyant material suitable for pottingthe insides of unmanned oceanographic probes.

Other objects and advantages will appear from the following descriptionof an example of the invention, and the novel features will beparticularly pointed out in the appended claims.

FIG. l illustrates graphically the relationship between compressivestrength of a particular cured resin and the parts of hardener per partsof uncured resin, and

FIGS. 2 and 3 illustrate a preferred technique of forming -a block ofsyntactic foam of a selected geometry.

I have discovered that a specific syntactic foam fabricated by aparticular technique is exceptionally suited to deep sea submergenceapplications. Syntactic foam is a material consisting of a resinousplastic matrix containing low density hollow granular ller. In thisinvention the iiller consists of high strength, approximately spherical,microscopic hollow glass grains, average particle density not over .45,wall thickness approximately 1.8 microns, 2090 microns outside diameter,a product which has been marketed commercially. The ller is ernbedded ina particular epoxy resin matrix. A composition 3 for the resin componentof the syntactic foam that I discovered to have the best properties isas follows:

Parts by wt. (a) The reaction product of epichlorohydrin with bis(4-hydroxyphenyl) dimethyl methane which is an epoxy resin. The resinshall have an epoxide equivalent in the range 175-210 and shall have aviscosity as low as is available but no higher than 5000 centipoises atC. As a practical matter the lowest viscosity of available commercialmaterials is somewhat less than 5000 centipoises at 25 C. 100 (b) Ahardener consisting of methylbicyclo [2,2,1] heptene 2,3-dicarboxylicanhydride isomers 100-104 (c) An eccelerator consisting ofbenzyldimethylamine(3) 1 Other hardeners were not satisfactory eitherbecause a syntactic foam of lower strength resulted or because ofdiiculties in handling; many of the acid type hardeners are solid atroom temperature and require preheating for use.

The resin cured according to the temperature schedule described below,and without filler, has a uniaxial compressive strength of approximately21,600 pounds per square inch, which is higher than that of any otherresin that might be used in syntactic foam for deep submergenceapplications. The range of hardener concentration is due to variationsamong commercially purchased hardeners. Actually the compositiondescribed is the optimum composition. The same materials in acomposition wherein the hardener concentration per hundred parts ofresin by weight is anywhere between 83-110 parts by weight, results in acured resin which has a compressive strength higher than that of anyother resin that might be used in syntactic foam for deep submergenceapplications. FIG. 1 shows the relationship between concentration ofhardener and uniaxial compressive strength.

The quantity of the above-recited accelerator may range from 0.2 to 1.0part by weight. Other accelerators may be used, e.g. trimethyl aminomethyl phenol 0.5 to 3.0 parts by weight, or dimethyl amino methylphenol 0.5 to 3.0 parts by weight, or alpha-methylbenzyl dimethyl amine1 to 3 parts by weight. The choice of accelerator and the quantityaffects the temperature and time of cure, as is well known to thoseskilled in the art. The cure time and temperature for the resin systemdescribed varies with specimen or casting size and geometry. Thefollowing cure schedule is preferred for a small specimen:

(a) 2 hours at 100 degrees C. (b) then 2 hours at 121 degrees C. (c)then postcured 16 hours at 177 degrees C.

Since the curing of the resin system is exothermic, larger specimensrequire a slower, more gradual cure schedule.

The other component of the syntactic foam is the microscopic hollowglass spheres. The syntactic foam has superior properties for deepsubmergence applications if the percentage of glass filler by weight iswithin the range of to 50 percent. The properties of the syntactic foamvaries with percentage of glass filler as follows. The density of thefoam is lower with a higher percentage of ller. However, compressivestrength, compressive modulus decrease while percentage water absorptionincreases with increased percentage of filler.

The water absorptiveness of the syntactic foam is reduced and thecompressive strength increased by the addition of less than one part byweight of one of the following coupling agents to the resin system.

(a) gamma-aminopropyltriethoxysilane (b)3,4-epoxycyclohexylethyltrimethoxysilane In the composition describedthe resin is somewhat stronger than the glass. The strength ofcommercially 4 marketed glass filler has been improved and it isforeseeable that a glass filler of the type described stronger than theresin described will become available.

In FIG. 2, there is shown a method of making the syntactic foamdescribed with optimum properties. A container 10 the serve as a moldhas an open end 12 and an opposite closed end 14 with an opening 16. Atransparent pipe stub 18 is sealed in the opening 16. The inner surfaceof the container in the direction between the bot tom 14 and the openend, all around the container, is continuous or straight and preferablyformed with enough draft to permit easy withdrawal of a casting formedin the container. The opening 16 is overlaid with a swatch of fiberglass cloth 20 attached to the surface around opening 16. The insidesurface of the container is waxed to render it relatively non-adherent.The container is supported level on any convenient means 22 thatprovides clearance for pipe stub 18, and completely filled with thehollow spherical granular glass filler 24. The container is vibrated,shaken, or tapped gently to compact the filler in the container and moreof the tller is added till level full. Then fiber glass cloth 26 isstretched across the open end 12 and cemented to the exterior surface. Astiff metal screen 28 is laid over the fiber glass cloth 26 and bentover the edge of the open end of the container.

The container is turned over and seated on spacer elements 30 in thebottom of a comparatively large waxed pan 32 thereby permitting fluid inthe pan to ow into the open end of the container. A hose 34 is coupledto the transparent pipe stub 18 and to a vacuum pump, not shown, througha conventional trap 36. The apparatus assembled as in FIG. 3 is locatedin a temperature controlled chamber wherein the temperature iscontinuously adjustable up to about 300 F. A supply of the resin systemmix is poured into the pan 32. The vacuum pump is set in operation. Itis advantageous to choose from among available resins that meet thepreviously recited specifications that resin having the lowest viscosityto facilitate the operation illustrated in FIG. 3. If the viscosity ofthe fluid appears to be too high the temperature of the chamber iselevated. Gelation is not hastened if the temperature remains well below200 F. The fluid rises into the container and after filling thecontainer rises into the transparent pipe stub or in the alternative,into a transparent sight tube, not shown, just past the pipe stub which,in the latter case, is not transparent. When this occurs the vacuum pumpis adjusted to reduce the pressure differential thereacross whereby thefluid level terminates in the glass stub. The pump is continued at thatsetting. Entrapped gas continues to escape thereafter. When the resin ispolymerized, the pump is shut down and the hose 34 disconnected. Some ofthe cured resin around the exterior of the container is chipped away.Then the casting in the container is readily removable. Since the hollowglass spheres are buoyant in the resin system mix, a thin layer of thecasting that was nearest the open end of the container may becomparatively free of the glass if there was any clearance in thecontainer for the glass to rise upwardly. That layer is cut away on aband saw.

The described method of making a cast block of the syntactic foamresults in a maximum ratio of glass to foam, namely, about percent glassfiller by volume and 35% resin system mix or about 40 percent glassfiller by weight. Also, there is minimum breakage of glass spheres inthe course of handling and forming the block, no dry or weaker areas,and minimum entrapped gas. It is not necessary to measure the filler;the quantity of filler used is the amount that fills the containercompactly. If the filler and resin system mix are combined in acontainer and mixed with a stirring paddle and then cured, a productthat is superior to other buoyancy means known in the art can beobtained but not as good and of as consistent quality as that obtainedby the method described. Stirring results in some breakage of the hollowglass spheres. If the mixture is not stirred the concentration of llernear the bottom of the mixture is very low. Some pockets of gas areentrapped, the filler is more dispersed and it is more difficult toobtain the optimum mixture.

It will be understood that various changes in the details, materials,and arrangements of parts (and steps), which have been herein describedand illustrated in order to explain the nature of the invention, may bemade by those skilled in the art within the principle and scope of theinvention as expressed in the appended claims.

I claim:

1. The method of making syntactic foam buoyancy material comprisingilling a straight-sided container that is open at one end, and that hasa central screened small opening in its end wall, with low densityhollow spherical granular glass ller free of resin coating,

vibrating the container to compact the ller in the container,

then adding enough additional filler to ll the container,

stretching a cloth across the open end of the container and securing thestretched cloth in place, then supporting the container, open enddownward, with the open end immersed in a uid epoxy resin system mlx,

coupling a vacuum pump to the screened opening of the container and walland operating the vacuum pump to withdraw the atmosphere from thecontainer and to ll the entire volume in the container not occupied bythe filler with fluid epoxy resin system mix, and curing the resin.

2. The method of making syntactic foam buoyancy material comprising:

ctlling a straight-sided container that is open at one end, and that hasa central screened small opening in its end wall, with hollow sphericalgranular glass liller that has average particle density not over 0.45and that is free of resin coating.

vibrating the container to compact the filler in the container,

then adding enough additional filler to ll the container,

stretching a cloth across the open end of the container and securing thestretched cloth in place,

then supporting the container, open end downward, and

immersed in a lfluid epoxy resin system mix consisting of 100 parts bylweight uncured reaction product of epichlorohydrin withbis(4hydroxyphenyl)di methyl methane and 100104 parts by weight ofmethylbicyclo (2,2,1) heptene 2,3 dicarboxylic anhydride isomers and 1part by weight of benzyldimethylamine (i3),

coupling a -vacuum pump to the screened opening of the container endwall and operating the vacuum pump to withdraw the atmosphere from thecontainer and to ll the volume in the container not occupied by thefiller with the uid epoxy resin system mix in an amount equal to to 70percent by weight of the composition,

and then curing the fiuid epoxy resin system mix.

References Cited UNITED STATES PATENTS 2,495,640 l/1950 Muskat 264-1022,774,108 12/19'56 Wyllie 264-102 2,903,389 9/1959 Fujita 264-1283,166,615 1/1965 Farrell 264--128 FOREIGN PATENTS 681,424 3/ 1964Canada.

ROBERT F. WHITE, Primary Examiner I. R. THURLOW, Assistant Examiner U.S.Cl. X.R.

